regcount – Display the allocation status of the TeX registers

Adds a macro \rgcounts which dis­plays the al­lo­ca­tion
sta­tus of the TeX reg­is­ters. The dis­play is writ­ten into the .log
file as it is a bit ver­bose. An au­to­matic call to
\rgcounts is done at \be­gin{doc­u­ment} and
\end{doc­u­ment}.